A case of nitridation, carburization and oxidation on a stainless steel

A case of corrosion was studied on stainless steel tubes, exposed to a nitriding, carburizing and oxidizing environment (mainly NH3 and CO2) at 390–450°C. Due to the high nitriding potential prior formation of internally nitrided layers occurs, at higher temperatures (> about 425°C) under precipitation of CrN in the layer and at lower temperatures under formation of the γN‐phase, i.e. austenite with high N‐content and expanded lattice. The latter process causes more severe corrosion, due to the high expansion, the stresses in the nitrided layers lead to bursting and repeated spalling of the scales. Carburization and oxidation are less important. The carburization is slower than nitridation, Fe3C formation is observed and carbon deposition. Also the oxidation by CO2 is slow and converts the nitrides and carbides formed before, to unprotective oxide flakes.